Eppo getStringAssignment: the default is a real fallback, and subject keys must be stable

TL;DR: call it with intent. Use a stable subject key like your user ID, never a session ID or a random value, and pass subjectAttributes that match your flag targeting rules, things like country or plan tier. Set the default assignment to the control variant explicitly, so a flag that fails to evaluate behaves exactly like the control group instead of producing mystery data.

Fix

1. call it with intent. Expected: this specific failure stops.

2. Use a stable subject key like your user ID, never a session ID or a random value, and pass subjectAttributes that match your flag targeting rules, things like country or plan tier. Expected: this specific failure stops.

3. Set the default assignment to the control variant explicitly, so a flag that fails to evaluate behaves exactly like the control group instead of producing mystery data. Expected: this specific failure stops.

4. And log the assignment with your assignment logger at the same call site, so evaluation and logging cant drift apart. Expected: this specific failure stops.

Details

Keep boolean, numeric, and JSON variants in their typed getters rather than parsing strings yourself.

When to use

You hit exactly this: Eppo getStringAssignment: the default is a real fallback, and subject keys must be stable in Eppo getStringAssignment.

When not to use

A different error, or the same symptom in a different tool. This page only covers the failure above.

Compatibility

Eppo getStringAssignment.

Official docs: the Eppo Node server SDK assignment docs document the call signature agents get wrong. getStringAssignment takes flagKey, subjectKey, subjectAttributes, and a default assignment. The default is what the SDK returns whenever no allocation matches, including misconfigured flags, so picking a meaningless default poisons your data. Subject keys must be stable user identifiers: a key that changes between sessions splits one user across variations and wrecks randomization. Docs: https://docs.geteppo.com/sdks/server-sdks/node/assignments/